Method of coating a liquid photographic emulsion on the surface of a support



Sept. 13, 1960 G. F. NADEAU 2,952,559 METHOD OF COATING A LIQUIDPHOTOGRAPHIC EMULSION ON THE SURFACE OF A SUPPORT Filed Nov. 1, 1956suhpoer GALE E NADEAU INVENTOR.

ATTORNEYS the support, or necking-in as it is known in the art.

United States Patent Office 2,952,559 Patented Sept. 13, 1960 METHOD OFCOATING A LIQUID PHOTO- GRAPHIC EMULSION ON THE SURFACE OF A SUPPORTGale F. Nadeau, Rochester, N.Y., assignor to Eastman Kodak Company,Rochester, N.Y., a corporation of New Jersey Filed Nov. 1, 1956, Ser.No. 619,721 7 Claims. (Cl. 117-34) This invention relates to a coatingmethod and, more particularly, to an improved method of coating liquidphotographic emulsions on supports.

In the process of coating webs of materials such as photographic papersor film supports with liquid photographic emulsion, ditficulty isexperienced as one attempts to increase the lineal coating speed of theweb. This is especially true in those instances where either scraper,extrusion or roller coating methods of application are involved. Onedifficulty which frequently occurs is the tendency for the coating topull away from the edge of The necking-in is aggravated by increases inthe consistency and viscosity of the coating being applied. It isimportant from the practical aspect to eliminate the necking-in of thecoating for the reason that excessive edging losses and problems withdrying and curing the heavy edges are otherwise presented. Manysuggestions have been offered for eliminating necking-in of the coatingsuch as the use of special edge shoes to lead the applied coating to thedesired width of the web and the addition of surfactants and other typesof spreading agents to the solution being coated. While these proposalslead to satisfactory results in certain instances, their use isseriously limited in the manufacture of photographic products because ofthe photographic effects of such added materials. Furthermore,surfactants have not proven very efiective in systems comprisingnon-aqueous solvents. Another serious problem encountered in coatingphotographic emulsions on support materials is photographic mottle whichresults from the non-uniform thickness of the application of thephotographic emulsion layer on the support. Because of the high surfaceresistivity of photographic support materials, it is possible to findrelatively high static Charges which vary in intensity and sign,occupying areas closely adjacent to each other. As a result of thisirregular static charge distribution on the surface of and in thesupport, disturbances in the coating bead frequently occur and theuniformity of the emulsion coating is therefore impaired.

It was in order to overcome or avoid these defects which occur with thecoating methods of the known prior art, that the research and devolpmentwas carried on,

which led to the coating method of the present invention. The coatingmethod comprises broadly the establishing of a uniform electrostaticcharge between the support and the photographic emulsion and applyingthe emulsion to the support.

The primary object of the present invention is, therefore, to provide amethod of coating a uniform thickness of photographic emulsion on asupport, thereby eliminating photographic mottle.

Another object of the present invention is to provide a method ofcoating a uniform thickness of photographic emulsion on a support inwhich the tendency for the phoi 'tographic emulsion to pull away fromthe edges of the support is eliminated.

Still another object of the present invention is to provide a method ofcoating a uniform thickness of photographic emulsion on a support whichcomprises applying a uniform electrostatic charge on the surface of thesupport and an electrostatic charge of opposite polarity to the liquidphotographic emulsion, and applying the charged emulsion to the chargedsupport.

Yet another object of the present invention is to provide a method ofcoating a uniform thickness of photographic emulsion on a support whichcomprises rendering the support electrically neutral, applying a uniformelectrostatic charge to the surface of the support, applying anelectrostatic charge of opposite polarity to the photographic emulsion,and applying the charged photographic emulsion to the charged support.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in the art from the followingdescription with reference to the drawing.

As was mentioned above, photographic mottle results from thenon-uniformity of the thickness of the photographic emulsion coatingwhich is applied to the support. The non-uniformity is caused by anirregular electrostatic charge distribution in and on the surface of thesupport being coated which causes disturbances in the coating bead ofthe liquid emulsion as well as causing an uneven flowing of the liquidemulsion on the surface of the support. Thus the first step of thecoating method of my invention is to render the surface of the supportelectri- Cally neutral. The electrostatic charges found to influence thecoating bead are of two general types. One is an irregular charge whichaccumulates within the support as a result of a polarization of some ofthe molecules of the support and which Will be hereinafter referred toas the polarization charge. The polarization charge is removed from thesupport by intimately contacting a grounded conductor to the opposedsurfaces of the support.

The drawing represents a diagrammatic view illustrating the apparatusused to practice the method of electrostatically applying a liquidphotographic emulsion on the As indicated in the drawing, this iseffected by passing the film support 21 between the grounded metalpressure rolls 23 and 25 which are mounted in opposed relation andbiased. by means of springs 27 and 29 respectively into contact with thesupport 21. The other type of electrostatic charge found to influencethe coating head, is the static charge which is present on the surfaceof the support. In order to discharge this surface charge, the filmsupport coming from between the pressure rolls 23 and 25 is passed overidler roll 31 and between a pair of opposed devices 33 and 35 forgenerating and directing a flow of ionized air onto the surface of thesupport 21. Thus the surface of the support 21 as it leaves the vicinityof the ionized air generating devices 33 and 35 is uniformlyelectrically neutral in charge.

I have found that the tendency of the liquid photographic emulsion topull away from the edge of the support is greatly reduced andpractically eliminated by applying a uniform electrostatic chargebetween the support and the liquid emulsion to be coated on the support.The application of the electrostatic charge influcnces the flowcharacteristics of the liquid emulsion by bringing about a reduction inthe surface tension thereof which is a direct result of a reduction inthe cohesive tension within the liquid. This phenomenon is not limitedto water and water solutions since any polarizable liquid responds in asimilar manner including those liquids having a specific conductivity ofthe magnitude, 10 ohms per 3 cc. When the electrostatic charge isapplied to the liquid emulsion, there is a tendency for the moleculeswithin the liquid to repel each other. This has the effect of reducingthe surface tension and increasing the flow rate of the liquid. Thus,when the charged liquid emulsion flows from an orifice onto the support,there is a strong tendency for the liquid particles to flow out to theedge of the support. The next step of the coating method of my inventionis, therefore, to apply a uniform electrostatic charge between thesurface of the support and the liquid photographic emulsion. Asindicated in the drawing, this is effected by directing a flow ofionized air of the desired polarity onto the surface of the support 21as by means of the device 37 which is similar to devices 33 and 35. Auniform electrostatic charge is thus distributed over the surface of thesupport 21 which remains on the support as it is conducted in contactwith coating roll .39 to the position at which the liquid emulsion iscoated thereon. An electrostatic charge of opposite polarity is appliedto the liquid emulsion by connecting a source of potential 41 to themetal orifice 43 of the hopper 45 through hich the liquid emulsion 47 ispassed for coating onto the support 21. The total potential differenceexisting between the charged support 21 and the .liquid emulsion 47should be less than 10,000 volts in order to avoid any possibility ofsparking which would cause fogging of the applied emulsions. I prefer tomaintain a potential difference of about 7,000 volts which is quitesufficient to give very satisfactory results.

By way of summary, the coating method according to my invention in itsmore specific aspects comprises, first, discharging the polarizationcharge within the support by passing the support in contact with a pairof opposed grounded metal rolls, discharging the static surface chargeon the support by directing a flow of ionized air onto the surface ofthe support, applying a uniform electrostatic charge to the support bydirecting a flow of ionized air of the desired polarity onto the surfaceof the support, applying an electrostatic charge of the oppositepolarity to the liquid photographic emulsion, and applying the .chargedphotographic emulsion to the charged support.

It should be understood that the coating method of my invention can bereadily utilized to coat any photographic emulsion on any known supportincluding photographic plates, film base and photographic printingpapers.

The utilization of a flow of ionized air to render a Web or other sheetmaterial electrically neutral or to distribute an electrical charge onthe surface thereof is well known in the art, and apparatus forgenerating such flows of onized air are disclosed in US. Patents1,678,869 which was granted to M. Morrison on July 31, 1938, and

2,497,604 which was granted to P. Henry et al. on February 14, 1950.

While only one embodiment of the present invention has been illustratedand specifically described, many modifications and variations arepossible and will be readily recognized by those skilled in the art and,therefore, the foregoing description is intended to be illustrative onlyand the scope of the invention is defined in the appended claims.

Having now particularly described my invention what I desire to secureby Letters Patent of the United States .and what I claim is:

l. A method of coating a liquid photographic emul- .sion on the surfaceof a support which comprises apply- .mg a uniform electrostatic chargeto said surface to facilitate the even spreading of said liquid emulsionon said surface, applying an electrostatic charge of opposite polarityto a supply of said liquid emulsion to facilitate the flowing of saidliquid emulsion on said surface, and flowing said'liquid emulsion in acontinuous liquid mass from said supply directly onto said surface, theelectric potential difference between said charge on said surface ;andsaid charge on said liquid Emulsion being less than:

that at which arcing occurs between said liquid emulsion and saidsurface.

2. A method of coating a liquid photographic emul sion on the surface ofa support which comprises rendering said surface electrically neutral,applying a uniform electrostatic charge to said surface to facilitatethe even spreading of said liquid emulsion on said surface, applying anelectrostatic charge of opposite polarity to a supply of said liquidemulsion to reduce the surface tension of said liquid emulsion andfacilitate the flowing thereof on said surface, and transferring saidliquid emulsion in a continuous liquid mass from said supply directly tosaid surface, the electric potential difference between said charge onsaid surface and said charge on said liquid emulsion being less thanthat at which arcing occurs between said liquid emulsion and saidsurface.

3. A method of coating a liquid photographic emulsion on the surface ofa support carrying polarization charges therein which comprisesneutralizing said polarization charges in said support by passing thesupport through the bite of a pair of opposed grounded metal rolls,rendering saidsurface electrically neutral by directing a flow ofionized air onto said surface, applying a uniform electrostatic chargeto said surface to facilitate the even spreading of said liquid emulsionon said surface, applying an electrostatic charge of opposite polarityto a supply of said liquid emulsion to facilitate the flowing of saidliquid emulsion on said surface, and transferring said liquid emulsionin a continuous liquid mass from said supply directly to said surface,the electric potential difference between said electrostatic charge onsaid surface and said electrostatic charge on said liquid emulsion beingless than that at which arcing occurs between said liquid emulsion andsaid surface.

4. A method of coating a liquid photographic emulsion on the surface ofa support which comprises first rendering said support electricallyneutral, applying a uniform electrostatic charge to said surface bydirecting a flow of ionized air of a given polarity onto said surfaceand thereby facilitating the even spreading of saidliquid emulsion onsaid surface, applying an electrostatic charge of opposite polarity to asupply of said liquid emulsion to facilitate the flowing of said liquidemulsion on said surface, and flowing said liquid emulsion in acontinuous liquid mass from said supply directly onto said surface, theelectric potential difference between said charge on said surface andsaid charge on said liquid emulsion being less than that at which arcingoccurs between said liquid emulsion and said'surface.

5. A method of coating a liquid photographic emulsion on the surface ofa support carrying polarization charges and static surface chargesthereon which comprises first discharging said polarization charges insaid support by passing the support through the bite of a pair ofopposed grounded metal rolls, rendering said static charges on saidsurface electrically neutral by directing a flow of ionized air ontosaid surface, applying a uniform electrostatic charge to said surface bydirecting a flow of ionized air of the desired polarity onto saidsurface to thereby facilitate the even spreading of said liquid emulsionon said surface, applying an electrostatic charge of opposite polarityto a supply of said liquid emulsion to facilitate the flowing of saidliquid emulsion on said surface, and transferring said liquid emulsionin a continuous liquid mass from said supply directly to said surface,the electric potential difference between said electrostatic vcharge onsaid surface and the electrostatic charge on said liquid emulsion beingless than that at which arcing occurs between said liquid emulsion andsaid surface. I

- port, the improvement which comprises applying to said surface auniform electrostatic charge havinga'polarity different from thepolarity of said liquid emulsiomand flowing the liquid emulsion fromsaid supply in a continuous liquid mass onto said surface whilemaintaining said charge thereon, the electric potential differencebetween the charge applied to said surface and the potential of saidliquid emulsion being less than that at which 5 arcing occurs betweensaid liquid emulsion and said surface.

7. The further improvement in the method according to claim 6 in which auniform electric potential difference is maintained between the chargedsurface and the 10 liquid emulsion.

References Cited in the file of this patent UNITED STATES PATENTSMorrison July 31, Chappell Aug. 25, Ransburg Nov. 1, Henry et al. Feb,14, Hart et al. Oct. 24, Miller May 1, Ransburg July 27, Miller Sept.20,

1. A METHOD OF COATING A LIQUID PHOTOGRAPHIC EMULSION ON THE SURFACE OASUPPORT WHICH COMPRISES APPLYING A UNIFORM ELECTROSTATIC CHARGE TO SAIDSURFACE TO FACILITATE THE EVEN SPREADING OF SAID LIQUID EMULSION ON SAIDSURFACE, APPLYING AN ELECTROSTATIC CHARGE OF OPPOSIDE POLARITY TO ASUPPLY OF SAID LIQUID EMULSION TO FACILITATE THE FLOWING OF SAID LIQUIDEMULSION ON SAID SURFACE, AND FLOWING SAID LIQUID EMULSION IN ACONTINUOUS LIQUID MASS FROM SAID SUPPLY DIRECTLY ONTO SAID SURFACE, THEELECTRIC POTENTIAL DIFFERENCE BETWEEN SAID CHARGE ON SAID SURFACE ANDSAID CHARGE ON SAID LIQUID EMULSION BEING LESS THAN THAT AT WHICH ARCINGOCCURS BETWEEN SAID LIQUID EMULSION AND SAID SURFACE.